An ion milling method is a processing method of milling a sample by using a sputtering phenomenon in which accelerated ions collide with the sample and the ions repel atoms or molecules. A mask as a shielding plate of an ion beam is mounted on a top surface of the sample to be processed, and a smooth sectional surface can be processed by sputtering a protruding portion from a mask end surface. This method is used for targets such as metal, glass, ceramic, electronic components, and composite materials. For example, this method is used for analyzing an internal structure, a sectional shape, film thickness evaluation, crystalline state, failure, or foreign-substance sectional surface of the electronic component. This method is used as a section sample creating method for performing acquisition of a morphological image, a sample compositional image, or a channeling image using various measurement devices including a scanning electron microscope, X-ray analysis, or crystallographic orientation analysis.
In the above-described ion milling device, an ion gun of a simple and small penning discharge type may be used as an ion gun. The ion gun of the penning discharge type has an advantage of achieving a high milling rate since energy of the ion beam is high. PTL 1 describes a method of constantly maintaining a current value of the ion beam emitted from the ion gun at a maximum value in order to maintain a high milling rate.